Scientific service

I. Introduction

ORBit accumulates years of experience on soil mapping and has the required expertise to carry out all aspects of a detailed spatial soil characterisation: planning (sampling strategy), implementation, interpretation, reporting and advice. We operate the latest generations of mobile proximal soil sensors, combined with precise GPS- positioning, geostatistical processing and GIS-cartography. This expertise is available as a scientific service.

The soil sensors we use are electromagnetic induction (EMI), ground penetrating radar (GPR) and magnetometry.

Some general features of our surveys:

  • Non-invasive, so non-destructive, i.e. no object goes into the soil.
  • Mobile, i.e. we drive a quad bike at walking speed while pulling the sensor (in a sled or mounted on a frame).
  • EMI and GPR are active sensors, i.e. they generate their own energy. As a result, they are less dependent on the environmental conditions than passive sensors (such as a magnetometer).
  • Very high spatial resolution. Since the sensors allow to register with a high sampling rate, we achieve measurable resolutions on the order of 1 observation per 0.2 m2 for EMI and magnetometry and 7.5 x 7.5 cm with GPR.
  • The maximum measurement depth can be adjusted according to the objective of the survey. Through the use of multiple coil configurations (EMI) or variable frequencies (GPR), the depth of investigation can vary between a few dm to > 6 m for EMI and > 3 m for GPR.
  • Highly accurate positioning: each measurement is attached to a differential corrected satellite positioning system (Omnistar, FLEPOS, base station) with a few cm as horizontal accuracy.

 

EMI-configuratie.jpg

 

II. How do our proximal soil sensors work?

1. Electromagnetic induction (EMI)

EMI is based on the principle that a primary magnetic field generated in a transmitter coil induces electrical eddy currents in conductive material, such as soil. These currents generate a secondary magnetic field. Both fields are captured by a receiving coil. This measurement makes it possible to determine simultaneously the electrical conductivity (EC) and the magnetic susceptibility (MS) of the affected material. The coil configuration determines the depth sensitivity profile.
Hence, by combining multiple coils, information about the depth of contrasting material can be obtained. EMI is sensitive to varying environmental conditions (moisture, salinity, temperature, electrical currents), but is in general quite robust. Ideal measurement conditions are at field capacity, but it is possible to obtain good measurements under very wet or dry conditions.

eddy currents.jpg

EC is indicative for various soil characteristics. Soil texture (especially the clay fraction) is the most dominant property. Additionally, salts, organic matter, compaction … all have an influence. MS is very sensitive to ferromagnetic minerals, organic matter (complexes with Fe) and anthropogenic disturbances such as excavations and heated material (brick, pottery, fires places… ). Both EC and MS are very sensitive to metal.


2. Ground penetrating radar (GPR)

A GPR antenna emits electromagnetic pulses of a certain frequency range. These pulses penetrate through the soil at a velocity which dependents mainly on its dielectric permittivity. If a contrasting object (e.g. a drain tube) or soil layer (e.g. a compacted zone) is present, then it will reflect a part of the wave energy. By measuring the time between the transmitted and returned pulse, the depth of this contrast can be determined. The wave frequency and the number of antennas determine the vertical and horizontal resolution of GPR measurements. Since water has a very high permittivity, it strongly reduces the penetration. Therefore, GPR measurements might be strongly limited in humid clayey conditions. On the other hand, dry sand is ideal for GPR.



werkinggpr.jpg

 

3. Magnetometry

A magnetometer allows a passive measurement of the strength of the Earth’s magnetic field at a specific location. Disruptions of this field can be seen as an “anomaly”. Such anomalies may be the result of soil disturbances (digging, excavations), or the presence of objects that affect the earth’s magnetic field (iron, brick foundations …). As a result of the inclination of the Earth’s magnetic field, these anomalies mostly appear as dipoles (positive and negative zones representing the object). By using multiple magnetometers together, it is the possibility to scan large areas in a short period of time.

Principe magnetometrie


III. Our proximal soil sensors

1. EMI

Dualem 1: consists out of two receiving coils at 1 m distance from the transmitter coil. The measurement volumes are 0-0.5 and 0-1.6 m. For this sensor, we have a manual foldable sled. This allows easy transportation and to use the sensor under conditions with limited manoeuvrability.

Dualem 21 & 21H
: consists out of four receiving coils at 1 and 2 m distance from the transmitter coil (see below). The measurement volumes are 0-0.5, 0-1, 0-1.6 and 0-3.2 m.
This EMI sensor is very suitable for most surveys related to soil mapping and for agricultural applications and archaeological prospection.

 

sensorconfig.jpg

dualem_1.jpg
Quqd setup
Dualem 421: consists out of six receiving coils at 1, 2 and 4 m distance from the transmitter coil. The measurement volumes are 0-0.5, 0-1, 0-1.6, 0-2, 0-3.2 and 0-6.4 m. These various measurement depths make the Dualem 421 very suitable for stratigraphic tomography (= the reconstruction of the soil layering) and mapping buried paleolandscapes.

Veldmeting Dualem 421S


Example EMI:
4 maps of EC measurements (below, left) and an EC depth-profile (stratigraphic tomography, right) according to the line on the 0-3 m EC map:

4 ec metingen

Ec tomografie

 


2. GPR

3D radar GPR: This type of GPR is of a “frequency-stepped continuous wave” type with a frequency range of 100-3000 MHz. Our antenna array consist of 13 antennas 7.5 cm apart (1 m scan width). In this way we can deliver a very fine horizontal resolution (7.5 x 7.5 cm) of reflectance measurements.

 

figure4.jpg
gprsetup.jpg

Stonehenge-jun2013-3d-radar1.jpg

 

GSSSI Scan Utility DF. This is a manual system working at two frequencies: 300 and 800 MHz. Two profiles are recorded simultaneously: a very detailed shallow profile and a coarser deep one. The system features a unique “blend mode” where both frequency measurements can be visualised in an integrated way

Terrein meting Gssi UtilityScan DFUtilityscan Gssi blend mode

 

3. Magnetometer

Sensys, 5 gradiometers. Both manually and motorized operated with a probe distance of 25 cm (1 m working width) or 50 cm (2 m working width).

Magnetometer manueelMagnetometer getrokken door quad